RDMA/mlx5: Separate create QP flows to be based on type

Move driver QP creation flow to separate functions to simplify
the create_qp() and allow future separation of create_qp_common()
to subtypes.

Link: https://lore.kernel.org/r/20200427154636.381474-8-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>